The difference in strength between boiler plates punched or drilled of full rivet size may be either greater or less than the difference in strength between unperforated plates of equal areas of fracture section.  When the metal plates are very soft and ductile, the operation of punching does no appreciable injury.  Prof.  Thurston says he has sometimes found it actually productive of increased strength; the flow of particles from the rivet hole into the surrounding parts causing stiffening and strengthening.  With most steel and hard iron plates the effect of punching is often to produce serious weakening and a tendency to crack, which in some cases has resulted seriously.  With first class steel or iron plates, punching is perfectly allowable, and the cost is twenty-five per cent. less than drilling; in fact, none but first class metal plates should be used in the construction of steam boilers.

In the original punching machines the die was made much larger than the punch, and the result was a conical taper hole to receive the rivet.  With the advanced state of the arts the punch and die are accurately fitted; that is to say, the ordinary clearance for a rivet of (say) three-fourths of an inch diameter, the dies have about three sixty-fourths of an inch, the punch being made of full rivet size, and the clearance allowed in the diameter of the die.

Take, for example, cold punched nuts.  Those made by Messrs. Hoopes & Townsend, Philadelphia, when taken as specimens of “commercial,” as distinguished from merely experimental punching, are of considerable interest in this connection, owing to the entire absence of the conical holes above mentioned.

When the holes are punched by machines properly built, with the punch accurately fitted to the die, the effect is that the metal is made to flow around the punch, and thus is made more dense and stronger.  That some such action takes place seems probable, from the appearance of the holes in the Hoopes & Townsend nuts, which are straight and almost as smooth as though they were drilled.

Therefore I repeat that iron or steel that is not improved by proper punching machinery is not of fit quality to enter into the construction of steam boilers.

Strength of punched and drilled iron bars.

Hoopes& Townsend.

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--------------+
Thickness of bar|Thickness outside |  Punched bars  |  Drilled bars  |
in inches.   |of hole in inches.|broke in pounds.|broke in pounds.|
----------------+------------------+----------------+-------

---------+
3/8 or 0.375  |   3/8 or 0.375   |     31,740     |     28,000     |
3/8 or 0.375  |   3/8 or 0.375   |     31,380     |     26,950     |
5/8 or 0.625  |   1/4 or 0.25    |     18,820     |     18,000     |
5/8 or 0.625  |   1/4 or 0.25    |     18,750     |     17,590     |
5/8 or 0.625  |  3/16 or 0.1875  |     14,590     |     13,230     |